This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This is a Phase II/III open-label study designed to evaluate the safety of GA-GCB therapy for patients with type I Gaucher disease who previously received imiglucerase. Patients will initially be screened for study participation based on their known medical histories. Patients must have received the same imiglucerase dose and dose regimen during the 6 months prior to study enrollment and must have had an overall imglulcerase treatment period of at least 30 months. To date, no standard dose has been defined or recommended for imiglucerase;therefore, an optimal or standard treatment dose for patients participating in this study could not be identified. Treatment at doses between 15 U/kg and 60 U/kg are most frequently used. In order to evaluate the safety of GA-GCB treatment at doses between 15 U/kg and 60 U/kg administered every other week, this study will enroll patients who have been treated with imiglucerase for at least 30 months at and between those doses. Patients must have been receiving the same imiglucerase dose and dose regimen during the 6 months prior to study enrollment. The 12 month treatment period with GA-GCB for patients enrolled into this study will allow evaluation of the safety of GA-GCB treatment at doses between 15 U/kg and 60 U/kg given every other week. The secondary objectives of the study are to evaluate changes in clinical parameters (hemoglobin concentration, platelet count, and liver and spleen volumes) throughout the 12 months of GA-GCB treatment. These parameters would be difficult to assess without considering additional efficacy data from patients previous treatment with imiglucerase. The proposed 12 month time frame for evaluation was determined based on clinical experience of expert consultants, which indicates that patients would likely have a reversal of improvement in clinical parameters after 3 months of receiving no ERT treatment. For each clinical activity parameter, the alternative hypothesis is that the mean change from Baseline (i.e., the end of imiglucerase treatment) to Month 12 is within the specified clinically significant levels for the parameters to be evaluated (where the population mean change from Baseline for hemoglobin is within 1 g/dL, the platelet count is within 20%, and the liver and spleen volumes are within 15%. This will be evaluated using a 2-sided 90% confidence interval for the true difference from Baseline for these clinical parameters. For example, efficacy of GA-GCB will be concluded if the confidence interval for the change from Baseline of hemoglobin is within the interval 1 to 1 g/dL. Our assumption is that the mean hemoglobin concentration will be essentially constant over the 12 month period. The power calculations for this study were based on results of TKT025, examining the Baseline mean values and the within patient change from Baseline standard deviations. As the study will be conducted at sites across the world, it is expected that a broad range of disease severity will be represented. Children who are a minimum of 2 years old who are not at high risk for type 3 Gaucher disease will be eligible for enrollment, as GA-GCB had an acceptable safety profile in TKT025. There was no antibody formation during 9 months of treatment in TKT025 and the first six months on TKT025EXT for a total of 15 treatment months. In addition, as has been previously discussed, there have been no drug related serious adverse events to date and only mild, transient infusion-related adverse events. Therefore, it is reasonable to treat patients 2 to 17 years old.